Optical connectors, including a housing holding the optical ferrule with an optical fiber, have end faces or surfaces that need to be a certain, optimum shape to prevent misalignments, air gaps, reflections, or scattering of light at interfaces at which optical fibers need to be coupled in an optical fiber link. Because of the precision needed, the end faces of optical connectors need to be inspected. Imperfections in the end faces of optical connectors are compounded by being summed with similar imperfections at other fiber junctions in the system and ultimately can lead to greatly increased light attenuation, lower signal to noise ratios, and lower system band width.
In some systems, to help preserve band width, connectors have been developed with angled cuts. In these types of arrangements, the ferrule is angle ground and then polished. Such angled polished connectors help to reduce incidents of back reflection. Back reflection is caused by two mating fiber ends that do not achieve physical contact with each other, creating a small air gap in the transmission path of the signal, which leads to back reflection of the laser light from the unmatched interface. To assure the necessary physical contact between two mating fibers, the ends can be angled and polished, as described above, and factors such as radius of curvature, fiber height, and apex offset of the polish with respect to the center of the fiber is controlled. These types of parameters will help to insure physical contact between the fibers, thereby minimizing loss and back reflection.
Before these connectors are installed in the field, they need to be inspected to measure, for example, the radius of curvature, the apex offset, and the fiber height. Various inspection machines have been developed for providing this type of testing. In general, these machines utilize interferometer based systems. One machine is commercially available from Direct Optical Research Company (DORC) of Phoenix, Ariz. The DORC machine uses an interferometer with a parfocal, parcentral zoom lens to inspect and measure an end surface of an optical fiber connector. DORC machines support the connector on a stage that is movable about three axes (X, Y, and Z axes) and the interferometer is set to a microscope mode. Controls of the stage along the X, Y, and Z axes are adjusted to bring the end surface into focus and alignment with the optical axis of the interferometer. The DORC machine also has a tilt stage that can be oriented to facilitate measurement of an angle on the end surface or protrusion or undercut of fiber with respect to the ferrule of a connector without readjusting the main stage. Further details about the DORC machine are described in U.S. Pat. No. 5,459,564 issued Oct. 17, 1995, and U.S. Pat. No. 6,215,555 issued Apr. 10, 2001, each of which is incorporated by reference herein.
Improvements in methods and apparatus for inspecting connector ends are desirable.